1. Field of Invention
The present invention relates to a method for producing a single silicon substrate having an insulation layer.
2. Description of Related Art
It is well known in the art that a substrate including a thin single crystal silicon layer provided on an insulation layer is preferable as compared to a bulky single crystal silicon substrate when producing an integrated circuit. This is due to the fact that the insulation layer separates and shields elements used in the integrated circuit.
A single crystal silicon semiconductor substrate having an electrically insulated layer and a surface single crystal silicon layer in which the elements are formed is generally called an SOI (Silicon On Insulator) substrate. This type of semiconductor substrate is produced by either a wafer bonding method or an oxygen-ion implantation method.
When using a wafer bonding method, an oxide layer is first formed on the single crystal silicon substrate. Then, a wetted second single crystal silicon substrate is stock to to the first silicon substrate, such that a thin active silicon layer is formed by the wetted second single crystal silicon substrate. The oxygen-ion implantation method, which is known as SIMOX (Separation by IMplanted Oxygen), is preferable as compared to the wafer bonding method when forming a thin-film active silicon layer. SIMOX forms a buried oxide layer inside of the silicon substrate by allowing Si and O to react in a high temperature annealing process (1,100.degree.-1,200.degree. C.) after implanting a high dose of oxygen ions (.sup.16 O.sup.+) into the single crystal silicon substrate.
The SIMOX method is the preferred method given that it produces a single crystal silicon substrate with an active layer having an uniform thickness without having to bond two wafers. However, even with the SIMOX method there are certain disadvantages.
Referring to FIG. 9, when a high quality substrate 1 is used in the SIMOX method a surface single crystal silicon 2 will generally have a low dislocation density. A surface single crystal silicon with a low dislocation density, as will be described below, adversely affects the electrically insulated state of a buried oxide layer 3 produced by oxygen ions. For example, a surface single crystal 2 with a low dislocation density will, upon implantation with oxygen ions, produce a buried oxide layer 3 with a small thickness which is approximately 80-90 nm. Thus, if a particle 10 attaches on the surface single crystal silicon 2 when oxygen ions are implanted, the buried oxide layer 3 will not be complete because of the masking effect produced by the the particle 10. In fact, after the annealing process, the incomplete buried oxide layer 3 produces a buried oxide layer 5 with a pinhole 9. As a result, an inferior electrical insulation layer is formed.
Reference 6 in FIG. 9 indicates a surface oxide layer which is formed after the annealing process.
FIG. 10 charts a correlation between the dose of the oxygen ions and the dislocation density in the surface single crystal silicon layer 2 (see J. Mater. Res., Vol. 8, No. 3, Mar. 1993 pp. 523-534). A a thickness of the buried oxide layer 3 can be increased by increasing the dose of the oxygen ions. Where the dose of oxygen ions is increased to a range of 1.0.times.10.sup.18 /cm.sup.2 -2.0.times.10.sup.18 /cm.sup.2, a crystal defect or dislocation density of single crystal silicon layer 2 is increased. Particularly, a dose of oxygen ions in over 1.5.times.10.sup.18 /cm.sup.2 results in a sudden increase in the dislocation density of the single crystal silicon layer 2.
However, referring to FIG. 11, when the dose of the oxygen ions is in a range of 0.5.times.10.sup.18 /cm.sup.2 -0.9.times.10.sup.18 /cm.sup.2 (i.e., the range of the non-increasing crystal defect density), a breakdown strength of the electric field in the buried oxide layer 3 falls between a range of 0-1 MV/cm which is considered small Thus, the electric insulation property of the resulting oxide layer will be low.
Also, in the SIMOX substrate having a thin buried oxide layer 3, irregularity of an interface between the surface single crystal silicon 2 and the buried oxide layer interface produces an undesirable roughness. Specifically, the interface generally has a square root mean roughness (Rms) which is over 2 nm. This undesirable roughness can be sufficiently removed by spending a large amount of time on the annealing process, however, this is impractical n view of costs.
Accordingly, in view of the foregoing problems, it is an object of the present invention to provide an SOI substrate which can increase a thickness of its buried oxide layer without increasing the dose of implanted oxygen ions to prevent an increase of crystal defect in a surface single-crystal silicon layer. It is another object of the present invention is to provide a method of producing the SOI substrate capable of decreasing a pinhole density in the buried oxide layer. It is yet another object of the present invention is to provide a method of producing the SOI substrate which improves the roughness of the buried oxide layer interface. It is still another object of the present invention to provide an SOI substrate in which the crystal-defect density of the surface silicon layer is small, the buried oxide layer is increased more than a theoretical buried oxide layer thickness calculated from the dose of the implanted oxygen ions, the generation of pinholes is small, and the roughness of the buried oxide layer interface is improved.